Method for increasing the quality of graphite balls

ABSTRACT

The invention provides a method for increasing the quality of graphite balls. The method comprises melting molten iron in an electric furnace, increasing the sulfur content in the molten iron during the melting process, and adding rare earth in the electric furnace or in a nodularizing ladle; after the molten iron is completely melted, pouring the molten iron into the nodularizing ladle and nodularizing; and after nodularization, adding ferromanganese to a transfer ladle. In the present invention, sulfur is added to molten iron in advance, and rare earth is added to a nodularizing ladle previously, so that a large number of dispersed rare earth sulfide particles are formed in the molten iron during the nodularization process. Rare earth sulfide particles serve as the nuclei of graphite crystallization to increase the number of graphite balls, and improve the roundness of graphite balls.

FIELD OF THE INVENTION

The present invention relates to the technical field of metallurgy andcast iron alloys, and particularly to a method for increasing thequality of graphite balls.

DESCRIPTION OF THE RELATED ART

Nodular cast iron is widely used, and the roundness and size of thegraphite balls in nodular cast iron are critical factors affecting itsperformance. It is well known that sulfur is a main factor causing poornodularization of graphite, and the molten iron is required to have asulfur content that is as low as possible to obtain round graphiteballs. Where fine graphite balls are desired, ferrosilicon is requiredfor inoculation. The nodularization allows graphite to grow spherically,and the inoculation increases the nucleation rate of graphite, ensuringthe number and roundness of graphite balls. Therefore, the traditionalnodularization technology includes desulfurization, and thennodularization and inoculation. For nodular cast iron with a thick andlarge cross-section, due to the long solidification time of molten ironafter inoculation, the inoculation effect is gradually deteriorated, asshown by a decrease in the number of graphite balls per unit area ofcast iron, the size of graphite balls is increased, and the degree ofnodularization becomes worse, affecting the mechanical performances ofcastings.

Therefore, for nodular cast iron with a thick and large cross-section,to increase the number of graphite balls and improve the roundness ofgraphite balls, generally a multi-stage inoculation technology includingladle inoculation, ladle-to-ladle inoculation, in-stream inoculation,and in-mould inoculation is employed. The process is cumbersome, anddifficult to control, and the nodularization effect is unstable.

Therefore, a method for producing nodular cast iron is desired, to solvethe above problems.

SUMMARY OF THE INVENTION

To solve the above technical problems, the present invention provides amethod for increasing the quality of graphite balls. In the method ofthe present invention, sulfur and rare earth are added in molten iron inadvance, to produce a large amount of rare earth sulfide that forms alarge number of nuclei of graphite crystallization, and thennodularization is performed, to extend the time of inoculation fade, andallow nodular cast iron with a thick and large cross-section to havegood nodularization effect.

An object of the present invention is to provide a method for increasingthe quality of graphite balls. The method specifically comprises thefollowing steps: increasing the sulfur content in base molten iron inadvance, and adding rare earth to a nodularizing unit previously, sothat dispersed rare earth sulfide particles are formed during thenodularization process, wherein the rare earth sulfide particles serveas the nuclei of graphite crystallization to increase the number ofgraphite balls, and improve the roundness of graphite balls.

In an embodiment of the present invention, the nodularizing unit is anodularizing ladle.

In an embodiment of the present invention, the rare earth is selectedfrom the group consisting of cerium, a lanthanum-cerium alloy and acerium-iron alloy and any combination thereof.

In an embodiment of the present invention, the rare earth is added tothe nodularizing unit previously, and the molten iron is poured into thenodularizing unit and nodularized, wherein the rare earth covers thesurface of a nodularizer.

In an embodiment of the present invention, the temperature change in thenodularization process is such that the molten iron is heated to 1500°C., held for 4 min, and then cooled to 1450° C. at which the molten ironis poured into the nodularizing ladle and nodularized.

In an embodiment of the present invention, the sulfur content isincreased by adding sulfur or ferric sulfide.

In an embodiment of the present invention, the rare earth accounts for0.01-0.08 wt % of the molten iron.

In an embodiment of the present invention, the sulfur content in thesulfur-increased molten iron is 0.03-0.07 wt %.

In an embodiment of the present invention, the raw material of the basemolten iron is selected from steel scrap and recycled scrap; the steelscrap is selected from carbon steel and/or alloy steel. The sources ofsteel scrap include, but are not limited to, scraps of stamping parts,for example, scraps of automobile stamping parts. The use of steelscraps as a raw material can avoid the hereditary effects of pig iron asa raw material.

In an embodiment of the present invention, the steel scrap in the rawmaterial accounts for 50-100 wt %, and the recycled scrap accounts for0-50 wt %.

In an embodiment of the present invention, the method further comprisesadding ferrosilicon, and a recarburizer to the base molten iron, to givea carbon content of 3.6-4.0 wt % and a silicon content of 1.8-2.1 wt %in the molten iron.

In an embodiment of the present invention, the method further comprisesadding ferromanganese to the molten iron after nodularization, whereinthe ferromanganese has a particle size of 5 to 15 mm.

In an embodiment of the present invention, the manganese content in themolten iron is 0.4-0.6 wt %.

In an embodiment of the present invention, the method for increasingnumber of graphite balls and improving the roundness of graphite ballsin nodular cast iron comprises the following operations:

Steel scrap and recycled scrap are used as main raw materials and meltedto produce nodular cast iron in During melting, an electric furnace.Ferrosilicon, a recarburizer, sulfur powder (or ferric sulfide), andrare earth are added, where the molten iron is controlled to have acarbon content of 3.6-4.0 wt %, a silicon content of 1.8-2.1 wt %, asulfur content of 0.03-0.07 wt %, and a rare earth content of 0.01 wt%-0.08 wt %. The method comprises specifically the following steps:

-   -   (1) Waste steel tube is used as raw material; a material list is        formulated, and entered into an automatic material weighing        system; and the materials are automatically weighed.    -   (2) Waste steel tube is added to an electric furnace, then        ferrosilicon and a recarburizer are added to the base molten        iron, and the sulfur content in the base molten iron in the        electric furnace is increased by adding a sulfur increasing        material selected from sulfur power (or ferric sulfide). After        the molten iron is melted, a sample is taken to analyze the        contents of various elements. The contents of various auxiliary        materials are adjusted to allow the molten iron to have a carbon        silicon, manganese, and sulfur content reaching the above        contents.    -   (3) The molten iron is heated to 1500° C., held for 4 min, and        then cooled to 1450° C. at which the molten iron is poured into        a nodularizing ladle previously added with a nodularizer and        nodularized, where rare earth is added to the nodularizing ladle        previously.    -   (4) After nodularization, FeMn68 ferromanganese is added to a        transfer ladle, where the manganese content is controlled to        0.4-0.6%. Then, a nodular cast iron specimen of 200 mm×200        mm×200 mm is casted.

Compared with the prior art, the technical solution of the presentinvention has the following advantages:

In view of the problems of reduced number of graphite balls and poorroundness of graphite balls, caused by inoculation fade during thenodularization of nodular iron castings with a thick and largecross-section, the traditional ideal of desulfurization and thennodularization and inoculation in the production of nodular cast iron isoverturned in the present invention. Instead, sulfur and rare earth areadded in molten iron in advance, to produce a large amount of rare earthsulfide that forms a large number of nuclei of graphite crystallization,and then nodularization is performed, to extend the time of inoculationfade, and allow nodular cast iron with a thick and large cross-sectionto have good nodularization effect.

Sulfur in molten iron can react with rare earth to form rare earthsulfide and also react with manganese to form manganese sulfide; andlarge manganese sulfide is difficult to be used as the nuclei ofgraphite nucleation, and reduces the mechanical performances of thematerial. Therefore, in the present invention, sulfur and rare earth areadded before nodularization, to form fine rare earth sulfide acting asthe nuclei of graphite balls. After nodularization, ferromanganese isadded, to reduce the formation of manganese sulfide inclusions.

By using the method for increasing the quality of graphite ballsaccording to the present invention, the obtained thick and largecastings of 200 mm×200 mm×200 mm have a nodularization rate of graphiteballs increased by 20% or more at the surface and in the core, adiameter of graphite balls increased by 1 grade, and a number ofgraphite balls per unit area increased by 50% or more.

BRIEF DESCRIPTION OF THE DRAWINGS

To make the disclosure of the present invention more comprehensible, thepresent invention will be further described in detail by way of specificembodiments of the present invention with reference the accompanyingdrawings, in which:

FIG. 1 schematically shows a sampling position in Comparative Example 3of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be further described below with reference tothe accompanying drawings and specific examples, so that those skilledin the art can better understand and implement the present invention;however, the present invention is not limited thereto.

The materials such as ferrosilicon, the recarburizer, ferrosulfur, therare earth alloy, and the nodularizer etc. used in the present inventionare all commercially available.

To enhance the effect of nodularization and inoculation of nodular castiron, the present invention provides a method for increasing the qualityof graphite balls. The method comprises the following steps.

Steel scrap and recycled scrap are used as raw materials and melted toproduce nodular cast iron. The sources of steel scrap include, but arenot limited to, scraps of stamping parts, for example, scraps ofautomobile stamping parts. The steel scrap may be carbon steel, alloysteel or a mixture of thereof. The use of steel scraps as a raw materialcan avoid the hereditary effects of pig iron as a raw material.

After the steel scrap and recycled scrap are melted, ferrosilicon and arecarburizer are added to adjust the components in molten iron, and thesulfur content in the base molten iron in the electric furnace isincreased to adjust the sulfur content in the base molten iron to0.03%-0.07%. Rare earth is added before nodularization, to allow themolten iron to have a cerium content reaching 0.01%-0.08%, and amanganese content of 0.4-0.6 wt %. After nodularization, ferromanganeseis added to the molten iron, and then a casting is formed by castingaccording to a conventional procedure.

EXAMPLE 1

This example provides a method for increasing the quality of graphiteballs. The method comprises the following steps:

-   -   (1) Waste steel tube and recycled scrap of nodular cast iron are        used as main raw materials and melted in an electric furnace,        where the waste steel tube and recycled scrap each account for        50 wt %. Ferrosilicon, a recarburizer, and ferrosulphur are        added during the melting process, where the molten iron is        controlled to have a carbon content of 3.6 wt %, a silicon        content of 2.1 wt %, and a sulfur content of 0.03 wt %.    -   (2) In this example, waste steel tube and recycled scrap of        nodular cast iron are used as raw materials, a material list is        formulated, and entered into an automatic material weighing        system; and the materials are automatically weighed.    -   (3) The waste steel tube and recycled scrap of nodular cast iron        are added to the electric furnace at a ratio of waste steel tube        to recycled scrap of nodular cast iron of 1:1. Then, FeSi75        ferrosilicon, the recarburizer, and FeS40 ferrosulphur are        added. After the molten iron is melted, a sample is taken to        analyze the contents of various elements. The contents of        various auxiliary materials are adjusted to allow the molten        iron to have a carbon, silicon, and sulfur content reaching the        above contents.    -   (4) A nodularizer is added to a nodularizing ladle, and Ce        covers the nodularizer, where the amount of cerium added is        calculated according to a content of 0.02 wt % in the molten        iron.    -   (5) The molten iron is heated to 1500° C., held for 4 min, and        then cooled to 1450° C. at which the molten iron is poured into        the nodularizing ladle and nodularized.    -   (6) After nodularization, FeMn68 ferromanganese is added to a        transfer ladle, where the manganese content is controlled to 0.4        wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm        is casted.

EXAMPLE 2

This example provides a method for increasing the quality of graphiteballs. The method comprises the following steps:

-   -   (1) Waste steel tube and recycled scrap of nodular cast iron are        used as main raw materials and melted in an electric furnace,        where the weight ratio of the waste steel tube and the recycled        scrap is 7:3. Ferrosilicon, a recarburizer, and ferrosulphur are        added during the melting process, where the molten iron is        controlled to have a carbon content of 4.0 wt %, a silicon        content of 1.8 wt %, and a sulfur content of 0.07 wt %.    -   (2) In this example, waste steel tube and recycled scrap of        nodular cast iron are used as raw materials, a material list is        formulated, and entered into an automatic material weighing        system; and the materials are automatically weighed.    -   (3) The waste steel tube and recycled scrap of nodular cast iron        are added to the electric furnace, where the weight ratio of the        waste steel tube and the recycled scrap is 7:3. Then, FeSi75        ferrosilicon, the recarburizer, and FeS40 ferrosulphur are        added. After the molten iron is melted, a sample is taken to        analyze the contents of various elements. The contents of        various auxiliary materials are adjusted to allow the molten        iron to have a carbon, silicon, and sulfur content reaching the        above contents.    -   (4) A nodularizer is added to a nodularizing ladle, and a        lanthanum-cerium alloy covers the nodularizer, where the amount        of the lanthanum-cerium alloy added is calculated according to a        content of 0.08 wt % in the molten iron.    -   (5) The molten iron is heated to 1500° C., held for 4 min, and        then cooled to 1450° C. at which the molten iron is poured into        the nodularizing ladle and nodularized.    -   (6) After nodularization, FeMn68 ferromanganese is added to a        transfer ladle, where the manganese content is controlled to 0.4        wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm        is casted.

EXAMPLE 3

This example provides a method for increasing the quality of graphiteballs. The method comprises the following steps:

-   -   (1) Waste steel tube is used as main raw material and melted in        an electric furnace, where no recycled scrap is added.        Ferrosilicon, a recarburizer, sulfur powder, and a        lanthanum-cerium alloy are added during the melting process,        where the molten iron is controlled to have a carbon content of        3.8 wt %, a silicon content of 2.0 wt %, a sulfur content of        0.03 wt %, and a cerium content of 0.01 wt %.    -   (2) In this example, waste steel tube is used as raw material; a        material list is formulated, and entered into an automatic        material weighing system; and the materials are automatically        weighed.    -   (3) The waste steel tube is added to the electric furnace. Then,        FeSi75 ferrosilicon, the recarburizer, sulfur powder, and the        lanthanum-cerium alloy are added. After the molten iron is        melted, a sample is taken to analyze the contents of various        elements. The contents of various auxiliary materials are        adjusted to allow the molten iron to have a carbon, silicon,        manganese, sulfur, and cerium content reaching the above        contents.    -   (4) A nodularizer is added to a nodularizing ladle, and a        lanthanum-cerium alloy covers the nodularizer, where the amount        of the lanthanum-cerium alloy added is calculated according to a        content of 0.01 wt % in the molten iron.    -   (5) The molten iron is heated to 1500° C., held for 4 min, and        then cooled to 1450° C. at which the molten iron is poured into        the nodularizing ladle previously added with the nodularizer and        nodularized,    -   (6) After nodularization, FeMn68 ferromanganese is added to a        transfer ladle, where the manganese content is controlled to 0.6        wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm        is casted.

EXAMPLE 4

This example provides a method for increasing the quality of graphiteballs. The method comprises the following steps:

-   -   (1) Waste steel tube is used as main raw material and melted in        an electric furnace, where no recycled scrap is added.        Ferrosilicon, a recarburizer, sulfur powder, and cerium are        added during the melting process, where the molten iron is        controlled to have a carbon content of 3.9 wt %, a silicon        content of 1.9 wt %, a sulfur content of 0.07 wt %, and a cerium        content of 0.08 wt %.    -   (2) In this example, waste steel tube is used as raw material; a        material list is formulated, and entered into an automatic        material weighing system; and the materials are automatically        weighed.    -   (3) The waste steel tube is added to the electric furnace. Then,        FeSi75 ferrosilicon, the recarburizer, sulfur powder, and a        lanthanum-cerium alloy are added. After the molten iron is        melted, a sample is taken to analyze the contents of various        elements. The contents of various auxiliary materials are        adjusted to allow the molten iron to have a carbon, silicon,        manganese, sulfur, and cerium content reaching the above        contents.    -   (4) A nodularizer is added to a nodularizing ladle, and a        lanthanum-cerium alloy covers the nodularizer.    -   (5) The molten iron is heated to 1500° C., held for 4 min, and        then cooled to 1450° C. at which the molten iron is poured into        the nodularizing ladle previously added with the nodularizer and        nodularized,    -   (6) After nodularization, FeMn68 ferromanganese is added to a        transfer ladle, where the manganese content is controlled to 0.5        wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm        is casted.

EXAMPLE 5

This example provides a method for increasing the quality of graphiteballs. The method comprises the following steps:

-   -   (1) Waste steel tube is used as main raw material and melted in        an electric furnace, where no recycled scrap is added.        Ferrosilicon, a recarburizer, ferrosulfur, and cerium are added        during the melting process, where the molten iron is controlled        to have a carbon content of 3.8 wt %, a silicon content of 2.1        wt %, a sulfur content of 0.05 wt %, and a cerium content of        0.08 wt %.    -   (2) In this example, waste steel tube is used as raw material; a        material list is formulated, and entered into an automatic        material weighing system; and the materials are automatically        weighed.    -   (3) The waste steel tube is added to the electric furnace. Then,        FeSi75 ferrosilicon, the recarburizer, sulfur powder, and a        cerium-iron alloy are added. After the molten iron is melted, a        sample is taken to analyze the contents of various elements. The        contents of various auxiliary materials are adjusted to allow        the molten iron to have a carbon, silicon, manganese, sulfur,        and cerium content reaching the above contents.    -   (4) A nodularizer is added to a nodularizing ladle, and a        cerium-iron alloy covers the nodularizer.    -   (5) The molten iron is heated to 1500° C., held for 4 min, and        then cooled to 1450° C. at which the molten iron is poured into        the nodularizing ladle previously added with the nodularizer and        nodularized,    -   (6) After nodularization, FeMn68 ferromanganese is added to a        transfer ladle, where the manganese content is controlled to 0.5        wt %. Then, a nodular cast iron specimen of 200 mm×200 mm×200 mm        is casted.

COMPARATIVE EXAMPLE 1

(1) In the comparative example, nodular cast iron is produced accordingto an existing conventional technology. That is, pig iron, steel scrap,and recycled scrap are used as raw materials, and the sulfur content iscontrolled to be low. Ferrosilicon is added during the melting processfor inoculation, followed by nodularization and casting of a nodularcast iron specimen of 200 mm×200 mm×200 mm. The carbon content, siliconcontent, and manganese content are controlled according to the ranges inthe examples; and the sulfur content is ≤0.03 wt %.

(2) The comparative example differs from the examples mainly in that themolten iron is subjected to sulfur-increasing treatment in the electricfurnace in the examples, rare earth is added to molten iron beforenodularization, and ferromanganese is added to molten iron afternodularization. In the comparative example, the sulfur content iscontrolled to be low, and no rare earth is added before nodularization.

(3) In the comparative example, pig iron, steel scrap and recycled scrapare used as raw materials specific proportion of the raw materials is 50wt % pig iron+30 wt % carbon steel scrap+20 wt % recycled scrap. Amaterial list is formulated, and entered into an automatic materialweighing system; and the materials are automatically weighed, added toan electric furnace, and melted.

(4) Ferrosilicon, ferromanganese, and a recarburizer are added duringthe melting process of molten iron. After melting, a sample is taken andanalyzed, and the components in molten iron are adjusted to have acarbon content of 3.6 wt %, a silicon content of 2.1 wt %, and amanganese content of 0.4 wt %; and the sulfur content in molten iron isdetected to be 0.02 wt %.

(5) The molten iron is heated to 1500° C., held for 4 min, and thencooled to 1450° C. at which the molten iron is poured into thenodularizing ladle previously added with the nodularizer andnodularized.

(6) After nodularization, the molten iron is casted to form a nodularcast iron specimen of 200 mm×200 mm×200 mm.

COMPARATIVE EXAMPLE 2

(1) In the comparative example, nodular cast iron is produced accordingto an existing conventional technology. Steel scrap, and recycled scrapare used as raw materials, and the sulfur content is controlled to below. Ferrosilicon is added during the melting process for inoculation,followed by nodularization and casting of a nodular cast iron specimenof 200 mm×200 mm×200 mm. The carbon content, silicon content, andmanganese content are controlled according to the ranges in theexamples; and the sulfur content is ≤0.02 wt %.

(2) The comparative example differs from the examples mainly in that themolten iron is subjected to sulfur-increasing treatment in the electricfurnace in the examples, rare earth is added to molten iron beforenodularization, and ferromanganese is added to molten iron afternodularization. In the comparative example, the sulfur content iscontrolled to be low, and no rare earth is added before nodularization.

(3) In the comparative example, steel scrap and recycled scrap are usedas raw materials, and specific proportion of the raw materials is 70 wt% carbon steel scrap+30 wt % recycled scrap. A material list isformulated, and entered into an automatic material weighing system; andthe materials are automatically weighed, added to an electric furnace,and melted.

(4) Ferrosilicon, ferromanganese, and a recarburizer are added duringthe melting process of molten iron. After melting, a sample is taken andanalyzed, and the components in molten iron are adjusted to have acarbon content of 4.0 wt %, a silicon content of 1.8 wt %, and amanganese content of 0.6 wt %; and the sulfur content in molten iron isdetected to be 0.015 wt %.

(5) The molten iron is heated to 1500° C., held for 4 min, and thencooled to 1450° C. at which the molten iron is poured into thenodularizing ladle previously added with the nodularizer andnodularized.

(6) After nodularization, the molten iron is casted to form a nodularcast iron specimen of 200 mm×200 mm×200 mm.

COMPARATIVE EXAMPLE 3

(1) In this comparative example, waste steel alone is used as rawmaterial, and the sulfur content is controlled to be extremely low.Ferrosilicon is added during the melting process for inoculation,followed by nodularization and casting of a nodular cast iron specimenof 200 mm×200 mm×200 mm. The carbon content, silicon content, andmanganese content are controlled according to the ranges in theexamples; and the sulfur content is ≤0.01 wt %.

(2) The comparative example differs from the examples mainly in that themolten iron is subjected to sulfur-increasing treatment in the electricfurnace in the examples, rare earth is added to molten iron beforenodularization, and ferromanganese is added to molten iron afternodularization. In the comparative example, the sulfur content iscontrolled to be extremely low, and no rare earth is added beforenodularization.

(3) In the comparative example, waste steel alone is used as rawmaterial, and specific proportion of the raw material is 100 wt % carbonsteel scrap. A material list is formulated, and entered into anautomatic material weighing system; and the materials are automaticallyweighed, added to an electric furnace, and melted.

(4) Ferrosilicon, ferromanganese, and a recarburizer are added duringthe melting process of molten iron. After melting, a sample is taken andanalyzed, and the components in molten iron are adjusted to have acarbon content of 3.8 wt %, a silicon content of 2.0 wt %, a manganesecontent of 0.5 wt %; and the sulfur content in molten iron is detectedto be 0.008 wt %.

(5) The molten iron is heated to 1500° C., held for 4 min, and thencooled to 1450° C. at which the molten iron is poured into thenodularizing ladle previously added with the nodularizer andnodularized.

(6) After nodularization, the molten iron is casted to form a nodularcast iron specimen of 200 mm×200 mm×200 mm.

The nodularization effects of graphite in each part of the nodular ironcastings of 200 mm×200 mm×200 mm obtained in the above examples andcomparative examples are shown in Table 1.

The test samples are taken from 4 surface positions and a central partof a cubic nodular iron casting of 200 mm×200 mm×200 mm, respectively.The 4 surface samples are numbered 1, 2, 3, and 4, and the centralsample is numbered 5, as shown in Table 1 below.

According to GB/T9441 “Metallographic Test for Spheroidal Graphite CastIron” in connection with image analysis, the nodularization rate, theball diameter and the number density of graphite in the sample aredetermined.

TABLE 1 Comparison of nodularization rate and number density of graphiteof Examples 1-4 and Comparative Examples 1-3 No. Test item E1 E2 E3 E4Average CE1 CE2 CE3 Average′ Sample Nodularization 86.5 87.2 83.1 88.586.3 71.3 66.2 69.1 68.9 1 rate (%) Ball diameter 5 5 5 5 5 4 4 4 4(grade) Number 159 172 152 169 163.0 97 116 98 103.7 density of graphite(balls/mm²) Sample Nodularization 86.3 85.4 82.7 89 85.9 66.7 67.3 70.168.0 2 rate (%) Ball diameter 5 5 5 5 5 4 4 4 4 (grade) Number 200 185163 190 184.5 88 106 91 95.0 density of graphite (balls/mm²) SampleNodularization 84.7 80.9 83.2 85.5 83.6 68.3 65.8 65.6 66.6 3 rate (%)Ball diameter 5 5 5 5 5 4 4 4 4 (grade) Number 175 162 155 176 167.0 103112 99 104.7 density of graphite (balls/mm²) Sample Nodularization 80.883.5 87.2 84.7 84.1 69.8 72.1 66.7 69.5 4 rate (%) Ball diameter 5 5 5 55 4 4 4 4 (grade) Number 147 163 181 177 167.0 117 98 109 108.0 densityof graphite (balls/mm²) Sample Nodularization 84.3 81.3 80.8 84.6 82.864.5 62.1 66.6 64.4 5 rate (%) Ball diameter 5 5 5 5 5 4 4 4 4 (grade)Number 181 166 172 178 174.3 131 105 111 115.7 density of graphite(balls/mm²)

In the above table 1, E1 refers to Example 1, E2 refers to Example 2, E3refers to Example 3, E4 refers to Example 4, and Average refers to theaverage of Examples 1-4; CE1 refers to Comparative Example 1, CE2 refersto Comparative Example 2, CE3 refers to Comparative Example 3, andAverage′ refers to the average of Comparative Examples 1-3. As can beseen from Table 1, the method for increasing the quality of graphiteballs has a nodularization effect that is obviously better than theexisting common methods. For a thick and large casting, thenodularization rate is increased by 20% or more, the diameter isincreased by 1 grade, and the number density of graphite is increased by50% or more.

Apparently, the above-described embodiments are merely examples providedfor clarity of description, and are not intended to limit theimplementations of the present invention. Other variations or changescan be made by those skilled in the art based on the above description.The embodiments are not exhaustive herein. Obvious variations or changesderived therefrom also fall within the protection scope of the presentinvention.

What is claimed is:
 1. A method for increasing the quality of graphiteballs, comprising steps of: increasing the sulfur content in base molteniron, and adding rare earth to a nodularizing unit, so that dispersedrare earth sulfide particles are formed during a nodularization process,wherein the rare earth sulfide particles serve as the nuclei of graphitecrystallization to increase the number of graphite balls, and improvethe roundness of graphite balls.
 2. The method according to claim 1,wherein the rare earth is selected from the group consisting of cerium,a lanthanum-cerium alloy and a cerium-iron alloy and any combinationthereof.
 3. The method according to claim 1, wherein the sulfur contentis increased by adding sulfur or ferric sulfide.
 4. The method accordingto claim 1, wherein the rare earth accounts for 0.01%-0.08% by weight ofthe base molten iron.
 5. The method according to claim 1, wherein thesulfur content in the sulfur-increased molten iron is 0.03-0.07 wt %. 6.The method according to claim 1, wherein the rare earth is added to thenodularizing unit previously, and the molten iron is poured into thenodularizing unit and nodularized, wherein the rare earth covers thesurface of a nodularizer.
 7. The method according to claim 1, wherein araw material of the base molten iron is selected from steel scrap andrecycled scrap; and the steel scrap is selected from carbon steel and/oralloy steel.
 8. The method according to claim 7, wherein the steel scrapaccounts for 50-100 wt %, and the recycled scrap accounts for 0-50 wt %in the raw material.
 9. The method according to claim 1, furthercomprising adding ferrosilicon, and a recarburizer to the base molteniron, to give a carbon content of 3.6-4.0 wt % and a silicon content of1.8-2.1 wt % in the molten iron.
 10. The method according to claim 1,further comprising adding ferromanganese to the molten iron afternodularization, and the ferromanganese has a particle size of 5 to 15mm.
 11. The method according to claim 10, wherein the manganese contentin the molten iron is 0.4-0.6 wt %.